Fetching details from the dictionary...

to be mentally slow; to be befuddled
to become engrossed in; to become absorbed in
to become frayed and dishevelled (of hair, grass, etc.); to be fluffy
テレビばかり見ていると頭がほうけてしまうよ。
If you do nothing but watch TV, your mind will go dull.
ほうけたふりで周囲の油断を誘うのが彼の戦略だ。
His strategy is to feign absent-mindedness to catch others off guard.
刺激のない生活を送るとだんだんほうけてくる。
If you lead a life without stimulation, you will gradually become mentally dull.